The vibration test industry, including all major aerospace firms, automotive manufacturers, electronics companies, and the like, has adopted use of various methods and systems to simulate vibration and shock environments for determining their products' effectiveness and longevity when subjected to these environmental extremes. Vibration testing is often used to develop vibration-tolerant designs; some use it to verify that a product will survive in its intended vibrating environment; others use it to screen out defective parts at an early stage in the manufacturing process. For instance, vibration and shock testing often involves the testing of electronic circuit boards adapted for use in spacecraft, airplanes, automobiles, etc. The boards are subjected to vibration testing, often at high frequencies, or high force levels, to determine whether or not they will survive the shaking required when the hardware is placed in use.
In the past, test fixtures in the form of "vibration tables" have been used for producing vibration and shock loads on an article under test. These vibration tables (which are also referred to as oil film tables, slip tables, or bearing tables) include a horizontal or vertical table on which the test article is mounted. The table is vibrated at a desired frequency, force level, and/or amplitude during testing. One such vibration test. fixture is manufactured and sold under the name T-Film Table by Team Corporation, South El Monte, Calif., the assignee of this application. This vibration test fixture is disclosed in U.S. Pat. No. 4,996,881, incorporated herein by this reference.
Vibration testing has traditionally been done with the test article restrained to move in a single axis. Recent studies show, however, that vibration testing in three mutually exclusive axes simultaneously can simulate real world conditions better than single axis testing. The present invention, in one embodiment, comprises a vibration test fixture adapted for single axis vibration; other embodiments of the invention comprise multiple-axis, multiple-degree-of-freedom vibration test fixtures.
Vibration frequency is selected to maximize the effectiveness of the testing. Transportation tests, for example, require frequencies from about 2 to about 500 Hz to simulate truck, rail or air transportation vibration. Screening and qualification tests may require that the frequency of vibration extend up to about 2,000 Hz for testing components with high natural frequencies. The vibration test generation and control input may be specified to be either sinusoidal motion, random motion, or to duplicate measured real-time waveforms. Vibration testing also can be applied at different force levels to meet certain maximum acceleration specifications.
The different types of test equipment currently available to produce vibration motion for a test article have limitations that require improvements. Electrodynamic shakers make up the largest percentage of purchased vibration force generators. They produce frequency response up to approximately 2,000 Hz and force ranges from about one pound to approximately 50,000 pounds. However, they are extremely expensive; they have relatively small specimen mounting areas; and they must often use a "head expander," a fixture that increases the mounting area for testing in the vertical axis, or a slip table or hydrostatic bearing system for testing in the horizontal axis. The electrodynamic shaker system adds considerable size to the usable specimen mounting surface, often taking up more room on the laboratory or production floor than is needed for the test object alone.
Hydraulic shakers have proved to be a viable alternative (compared with electrodynamic shakers) for all vibration testing, except those tests that require high frequency responses above about 500 to 1,000 Hz. Hydraulic shakers are physically much smaller than electrodynamic shakers, since the conversion of hydraulic power to vibrating motion can be accomplished with a much smaller mechanism. Hydraulic shakers also are much less expensive.
Head expanders, being physical devices, respond to certain driving frequencies by resonating. At each resonant frequency, the head expander deforms into a characteristic shape, and the frequency and shape together define that mode of vibration. The effect of the modes is to make the vibrating motion on the mounting surface of the head expander nonuniform. At a modal frequency, one location on the mounting surface may move less than the shaker input, while a different location may be moved more than is desired. Head expanders also add mass to the vibration test system, requiring much more force of the shaker than the test article alone.
A slip plate, and more precisely, the moving element of any horizontal vibration test fixture, has modes of vibration and suffers from the same deficiencies as head expanders. In addition to normal modes, the slip plate and shaker system is often long enough so that pressure or stress wave phenomena are observed to significantly degrade the uniformity of vibration of the slip plate surface. The observed phenomenon is that, when controlling the amplitude of a vibration test by monitoring the end of the slip plate, the front and center of the slip plate are often seen to have much lower amplitude over a wider range of frequencies. Conversely, if the test is controlled by monitoring the front of the slip plate, then the end is observed to have much higher amplitudes than desired.
These physical phenomena reduce the useful area of a head expander or a slip plate because it is not possible to obtain uniform vibration input over the entire surface.
The present invention provides a hydraulic vibration test fixture that overcomes the disadvantages of prior art electrodynamic shakers and hydraulic shakers. The test fixture avoids the cost and the space requirements and limitations of electrodynamic actuators, while achieving the higher frequency and load level capabilities normally associated only with electrodynamic shakers and not achieved by prior art hydraulic vibration test fixtures. Greatly improved uniformity of vibration amplitude across the useful surface area of the vibration table also is produced. Improvements in multiple-axis, multiple degree-of-freedom shakers are also provided.